Steve

It means it was designed as a BP Rifle but was proofed and regulated for a light load of smokeless. Probably a 325 grain bullet. It would carry the BP ballistics.

There are a lot of them around. I used to have a Patton from Scotland. I sold it to a fellow and he sold it to a PH in Zim who used full loads in it for Elephant. That is until he blew it off the face and ruined it.